In the computer workstation market, there is a demand for more computing power which requires more electric power. At the same time, it is advantageous to allow use of power from a standard 120V wall outlet. Customers are understandably reluctant to rewire offices with nonstandard outlets. Not only does this represent extra cost, but any time the workstation is moved, the rewiring must be repeated. A power supply with unity power factor input is an important step in addressing this need.
The average power obtained from an AC supply through an AC to DC converter is always something less than the product of the RMS voltage and RMS current. The ratio of the average power to the product of RMS voltage and current is known as the power factor. A typical switching power supply has an input power factor of 65%; this means that the power drawn from the utility line is 65% of the product of the voltage and current in the utility line and only 65% of what could be obtained with unity power factor.
The voltage supplied from an outlet is sinusoidal, but the system typically draws nonsinusoidal current. Unity power factor is obtained by assuring that a sinusoidal current, synchronized to the sinusoidal voltage, is drawn. Assuring that the current is sinusoidal also limits harmonic current, a feature required by some specifications.
One class of AC to DC converter which provides for power factors approaching unity is that of a full wave rectifier followed by a boost converter. In the boost converter, the output of the full wave rectifier is applied to a series connection of an inductor, diode and load capacitor. The charge carried by the load capacitor is the supply seen by the computer or other device utilizing the power. A shunting switch is coupled from a node between the inductor and diode to ground. The shunting switch is alternately opened and closed at a high frequency such as 40 kilohertz. When the switch is closed, the inductor is energized by current from the bridge rectifier to ground. When opened, the inductor relaxes and drives a current spike through the diode to charge the capacitor. In such a system, the duty cycle of the switch can be controlled such that the current closely follows the voltage applied from the bridge rectifier in waveform. The duty cycle of the shunting switch can be controlled by means of a feedback circuit to assure that the output voltage stored on the load capacitor is retained at a desired level. An example of such a boost converter can be found in U.S. Pat. No. 4,677,366 to Wilkinson et al.